1. Field of the Invention
This invention relates generally to nanotube fabrication and more specifically to manufacture of nanotubes containing boron, carbon and nitrogen.
2. Description of Related Art
Carbon tubes prepared by arc-discharge and having diameters on the order of nanometers have recently been synthesized. These tubes consist of cylindrical arrangements of carbon atoms. They can be produced in several ways, most commonly by arcing together two graphitic electrodes in a gas environment.
Carbon tubes can be synthesized in single-walled or multi-walled forms. The general apparatus for synthesis consists of an arc-discharge chamber. In the center of the chamber are two electrodes. The chamber is filled with a gas or gas mixture. An electric current is applied between the electrodes to form an arc. A deposit is formed inside the chamber which contains miniature carbon tubes. The deposit may be on one or both electrodes, and may also be on the inside walls of the chamber.
To optimize the yield or type of tubes desired, the type of electrodes, the type of gas, the nature of the current (a.c. or d.c.) and the voltage can be varied.
In general, the electrodes are made of graphite. They may also have dimples to hold metals to be vaporized, for example, iron. Or, they may have small cavity to be filled with either catalysts and/or graphite.
The type of gas and pressure inside the chamber affect the product synthesized. For example, 100 torr Argon was used to grow the first carbon tubes at the negative end of the electrode. S. Iijima, Helical Microtubules of Graphitic Carbon, Nov. 7, 1991, pp. 56-58, Nature, Vol. 354. Then, a mixture of 10 torr methane and 40 torr Argon was used to make single-shelled tubes with diameters of about one nanometer. S. Iijima and T. Ichihashi, Single-Shell Carbon Nanotubes of 1-nm Diameter, Jun. 17, 1993, pp. 603-604, Nature, Vol. 363. And, 100-500 torr Helium was used in cobalt catalyzed growth of carbon tubes with single-atomic-layer walls. D. S. Bethune, et al., Cobalt-Catalyzed Growth of Carbon Nanotubes with Single-Atomic-Layer Walls, Jun. 17, 1993, p. 605, Nature, Vol. 363.
The electric current depends on the size of the electrodes, their separation, and the gas pressure. It can be direct current (d.c.) or alternating current (a.c.), and has ranged from about 50 A to about 200 A for electrode diameters ranging from 1/4" to over 1".